1. Field of the Invention
The present invention relates to a wavelength selective switch module for compensating for characteristic changes of an optical switch due to temperature fluctuation or interannual fluctuation.
2. Description of the Related Art
WDM (Wavelength Division Multiplexing) is known as one of predominant means for constructing a large-capacity optical communications network. In recent years, traffic conveyed by the WDM scheme has been explosively increasing because of the widespread use of the Internet.
An optical cross-connect (OXC) system used in a backbone optical network based on the WDM scheme includes plural optical signal switching apparatuses that are interconnected by optical fibers. When the optical signal switching apparatus receives wavelength-multiplexed optical signals via the optical fiber, the optical signal switching apparatus switches routes of the optical signals for each wavelength, wavelength-multiplexes optical signals of a same route and transmits the multiplexed signal.
By using such optical cross-connect apparatus, when a failure occurs in an optical fiber that forms a communications route, the system can be recovered at high speed by instantaneously diverting traffic of the failed route to a standby optical fiber or to an optical fiber of another route. In addition to that, the optical cross-connect apparatus can edit optical paths for each wavelength.
A MEMS (Micro Electro Mechanical Systems) optical switch performs feedback control such that an optical output level becomes a desired level. When an initial value voltage is changed due to temperature characteristic change or interannual fluctuation, it is necessary to adjust the voltage by performing the feedback control such that inserting loss becomes minimum. Thus, there is a problem in that time required for switching becomes long.
When an optical amplifier is connected after the optical switch, level equalization is performed by using a VOA (Variable Optical Attenuation) function of the optical switch such that optical level of each wavelength input into the optical amplifier becomes the same. However, when the initial value voltage is changed, the VOA function does not work correctly, so that there is a case in which an optical level for a wavelength increases too much so that an optical surge occurs.
As described in Japanese Laid-Open Patent Application No. 2004-48187 (Patent document 1), in an optical switch using conventional MEMS, light sources are connected to every input port via a coupler, and feedback control is performed such that output level of the optical switch becomes a desired level to correct the deviation of the initial value voltage.
Japanese Laid-Open Patent Application No.2005-275094 (Patent document 2) discloses an optical switch including a control unit for changing an output optical level detected by an optical detection unit to a desired level.
Japanese Laid-Open Patent Application No.2005-195474 (Patent document 3) discloses an optical switch for compensating for wavelength deviation of calibration light based on calibration light wavelength error and calibration light output.
By using the conventional technique disclosed in the patent document 1, the deviation of the initial value voltage due to temperature characteristics or interannual variability of the MEMS optical switch can be corrected. But, since it is necessary to test all paths, it is difficult to increase speed for correcting the initial value voltage. In addition, since test light sources the number of which is the same as the number of the channels are necessary, there is a problem in that cost reduction and size reduction for the apparatus is difficult.
An object of the present invention is to provide a wavelength selective switch module that can reduce time for correcting the initial value voltage and that can realize cost reduction and size reduction, and that requires only one test light source.